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Discovery of high-amplitude X-ray variability in the Seyfert-LINER transition galaxy NGC 7589

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journal contribution
posted on 03.10.2012, 13:26 by W. Yuan, S. Komossa, D. Xu, Julian P. Osborne, Michael G. Watson, R.G. McMahon
We present the first result of a programme to search for large flux variations in the X-ray sources of the XMM Serendipitous Survey compared to previous ROSAT observations. An increase in X-ray flux by a factor >10 was discovered from the nucleus of the galaxy NGC7589 on a time-scale of less than 5 yr. The 0.4-10 keV XMM spectrum can be approximated by a power law with photon index of 1.7-1.8, though it seems to flatten above 5 keV, suggesting a possible complex model, such as partial covering or disc reflection. A classification based on an analysis of its optical spectrum places NGC 7589 in the Seyfert region, but close to the Seyfert-LINER (low-ionization nuclear emission-line region) borderline on the active galactic nucleus (AGN) diagnostic diagrams. We classify NGC 7589 as either Seyfert 1.9 or LINER I, in the light of the detection of a broad Halpha line, which makes NGC 7589 an AGN in the low-luminosity regime. We interpret the observed variability in terms of either changes in covering factor of absorbing gas in the AGN, or variability in the intrinsic X-ray luminosity. Should the latter be the case, the inferred Eddington accretion rate increased from the radiatively inefficient accretion-dominated regime to a value close to the putative critical value, at which a transition of the accretion mode is supposed to take place. This possibility presents a new prospect of studying accretion physics in the central black holes of external galaxies by direct observing changes of 'spectral state', as is common in stellar black hole binary systems.

History

Citation

Monthly Notices of the Royal Astronomical Society, 2004, 353 (3), pp. L29-L33 (5)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Blackwell Publishing on behalf of the Royal Astronomical Society

issn

0035-8711

eissn

1365-2966

Copyright date

2004

Available date

03/10/2012

Publisher version

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2004.08266.x/abstract

Language

en